INTRODUCTION. 23 



forms of the differentiated tissues throughout the animal kingdom are strikingly 

 alike. Striated muscle, with the characteristic property of sharp and powerful 

 contraction, is everywhere found ; the central nervous system in the inver- 

 tebrates is built upon the same type as in the highest mammals, and the 

 variations met with in different animals are probably but varying degrees of 

 perfection in the development of the innate possibility contained in primitive 

 protoplasm. It is not too much to say, perhaps, that were we acquainted with 

 the structure and chemistry of the ultimate units of living substance, the key 

 to the possibilities of the evolution of form and function would be in our 

 possession. 



Most interesting suggestions have been made in recent years as to the 

 essential molecular structure of living matter. These views are necessarily 

 very incomplete and of a highly speculative character, and their correctness or 

 incorrectness is at present beyond the range of experimental proof; never- 

 theless they are sufficiently interesting to warrant a brief statement of some 

 of them, as they seem to show at least the trend of physiological thought. 



Pfliiger, 1 in a highly interesting paper upon the nature of the vital pro- 

 cesses, calls attention to the great instability of living matter. He supposes 

 that living substance consists of very complex and very unstable molecules of 

 a proteid nature which, because of the active intra-molecular movement pre- 

 sent, are continually dissociating or falling to pieces with the formation of 

 simpler and more stable bodies such as water, carbon dioxide and urea, the act 

 of dissociation giving rise to a liberation of energy. " The intra-molecular 

 heat (movement) of the cell is its life." He suggests that in this living mole- 

 cule the nitrogen is contained in the form of a cyanogen compound, and that 

 the instability of the molecule depends chiefly upon this particular grouping. 

 Moreover the power of the molecule to assimilate dead proteid and convert it to 

 living proteid like itself he attributes to the existence of the cyanogen group. 

 It is known that cyanogen compounds possess the property of polymerization, 

 that is, of combining with similar molecules to form more complex mole- 

 cules, and we may suppose that the molecules of dead proteid when brought 

 into contact with the living molecules are combined with the latter by a pro- 

 cess analogous to polymerization or condensation. By this means the stable 

 structure of dead proteid is converted to the labile structure of living proteid, 

 and the molecules of the latter increase in size and instability. When living 

 substance dies its molecules undergo alteration and become incapable of ex- 

 hibiting the usual properties of life. Pfliiger suggests that the change may 

 consist essentially in an absorption of water whereby the cyanogen grouping 

 passes over into an ammonia grouping. Loew 2 assumes also that the dif- 

 ference between dead and living or active proteid lies chiefly in the fact that 

 in the latter we have a very unstable or labile molecule in which the atoms are 

 in active motion. The instability of the molecules he likewise attributes to 



1 Archiv fur die gesammte Physiologic, 1875, Bd. 10, p. 251. 



2 Ibid., 1880, Bd. 22 ; Loew and Bokorny : Die chemische Kraftquelle in lebenden Protoplasma, 

 Miinchen, 1882; Imperial Institute of Tokyo (College of Agriculture), 1894. 



